fix(gateway): broaden get_config 503 to any config-load failure

Address review feedback on the previous commit:

1. Narrow exception catch removed. The old contract returned 503 whenever
   `app.state.config is None`. The first cut only mapped
   `FileNotFoundError`, leaving `PermissionError`, YAML parse errors, and
   pydantic `ValidationError` to bubble up as 500. At the request boundary
   we treat any inability to materialise the config as "configuration not
   available" (503) and log the original exception so the operator still
   has the stack.

2. Removed the unused `request: Request` parameter and the matching
   `# noqa: ARG001`. FastAPI's `Depends()` does not require the dependency
   to accept `Request`; the only call site uses the no-arg form.

3. `backend/CLAUDE.md` boundary now lists the *reason* each field is
   restart-required (engine binding, singleton caching, one-shot
   `apply_logging_level`, etc.), not just the field name, so reviewers do
   not have to reverse-engineer the boundary themselves.

Tests parametrise four exception classes (`FileNotFoundError`,
`PermissionError`, `ValueError`, `RuntimeError`) and assert 503 for each.

Refs: bytedance/deer-flow#3107 (BUG-001)

backend/tests/test_gateway_config_freshness.py

@@ -20,6 +20,7 @@ import pytest
 from fastapi import Depends, FastAPI
 from fastapi.testclient import TestClient
 
+from app.gateway import deps as gateway_deps
 from app.gateway.deps import get_config
 from deerflow.config.app_config import (
     AppConfig,
@@ -108,3 +109,35 @@ def test_get_config_respects_test_set_app_config():
     app = _build_app()
     client = TestClient(app)
     assert client.get("/probe").json() == {"log_level": "warning"}
+
+
+@pytest.mark.parametrize(
+    "exception",
+    [
+        FileNotFoundError("config.yaml not found"),
+        PermissionError("config.yaml not readable"),
+        ValueError("invalid config"),
+        RuntimeError("yaml parse error"),
+    ],
+)
+def test_get_config_returns_503_on_any_load_failure(monkeypatch, exception):
+    """Any failure to materialise the config must surface as 503, not 500.
+
+    Bytedance/deer-flow issue #3107 BUG-001 review: the original snapshot
+    contract returned 503 when ``app.state.config is None``. The first cut of
+    this fix only mapped ``FileNotFoundError`` to 503, which left
+    ``PermissionError`` / ``yaml.YAMLError`` / ``ValidationError`` etc. bubbling
+    up as 500. Catch every load failure at the request boundary.
+    """
+
+    def _broken_get_app_config():
+        raise exception
+
+    monkeypatch.setattr(gateway_deps, "get_app_config", _broken_get_app_config)
+
+    app = _build_app()
+    client = TestClient(app, raise_server_exceptions=False)
+    response = client.get("/probe")
+
+    assert response.status_code == 503
+    assert response.json() == {"detail": "Configuration not available"}